<style>
    .panel-bg {
        background: rgba(0, 0, 0, 0.5);
        position: absolute;
        left: 0;
        top: 0;
        width: 100%;
        height: 100%;
        display: flex;
        align-items: center;
        justify-content: center;
    }

    .panel {
        min-width: 50%;
        min-height: 15em;
        max-width: 90%;
        background: gray;
        z-index: 99;
        display: flex;
        flex-direction: column;
        align-items: center;
        justify-content: space-around;
        padding: 1em;
    }
</style>

<dom>
    <div class="panel-bg">
        <div class="panel">
        </div>
    </div>
</dom>